Actually, we have both. All trains that regularly run around the City Circle and on the T1/4/9 have the same door spacings. V Sets have a different door spacing, but don't run through the city except for in trackwork, which is an arrangement that could be avoided if need be. So tick on the door spacing in the city stations.
As for the stopping accurately, our trains are good enough. There are numerous cities around the world that use platform screen doors on human driven systems. The solution to human inaccuracy? Wider doors that account for if a train is a bit off. Our trains aren't any more inaccurate than those overseas - you can see this for yourself by standing at Chatswood and watching trains arrive, and seeing how they line up with the door markers on the platform. Wide platform screen doors equals tick on that one too.
So really there's nothing preventing us from having platform screen doors on Sydney Trains stations (except stations like Sydney Terminal, Macarthur, Epping, Hornsby, etc that share platforms between Sydney Trains, V Sets, Endeavours, XPTs, historical trains, etc).
Platform screen doors won't solve dwell times though. They just lower the chances of some idiot walking onto the tracks.